1. Field of the Invention
The present invention generally relates to power steering system for vehicles. More particularly, the invention relates to a power steering system for vehicles of an electric type which produces auxiliary torque for steering by means of a steering servo unit using an electric motor.
2. Description of Relevant Art
In view of problems on the hydraulic type power steering system such as that the structure thereof is complicated, recent years have proposed a variety of electric type power steering systems for vehicles.
Exemplarily, in Japanese Patent Application Lay-Open Print No. 59-70257, laid open on Apr. 20, 1984, there is disclosed an electric power steering system for vehicles.
This electric power steering system for vehicles includes an input shaft as a steering shaft connected to a steering wheel, an output shaft interconnected through a universal joint with the input shaft and through a gear mechanism of a rack and pinion type with a tie rod of a steered wheel, an electric motor supplying auxiliary torque through a reduction gearing to the output shaft. A torque detection mechanism is disposed on the input shaft to detect steering torque acting on the input shaft, and a driving control circuit produces, based on a detection signal from the torque detection mechanism, a torque magnitude signal and a torque direction signal representing the magnitude and the direction of the steering torque acting on the input shaft, respectively, and to feed the electric motor with an armature current in proportional relation to the quantity to the torque magnitude signal and in accordance to the direction of conduction with the torque direction signal. The torque detection mechanism consists of a strain gauge sensor.
With such an arrangement, when the steering wheel is operated, the output shaft is applied with adequate auxiliary torque from the electric motor, so that the steering operation is facilitated.
In such electric power steering systems as according to the aforementioned Japanese Lay-Open Print, however, like the case of ordinary manual steering systems without assist power, there is employed a steering gear mechanism in which, when a steering wheel is rotated in either direction, clockwise for example, by a predetermined angle from a neutral position thereof, a rack, at the output side, is positioned at a corresponding one of the stroke ends thereof, keeping the steering wheel from being further rotated n the same direction. In general, the predetermined angle is approximately 540.degree., or corresponds to approximately one and half revolutions of the steering wheel.
In electric power steering systems of such type as according to the aforementioned Japanese Lay-Open Print, therefore, at the stroke ends of an output-side rack, that is, at both steering ends of a steering wheel, it is rather desirous to reduce or stop producing auxiliary torque by an electric motor. Such a desire is raised from the viewpoint that the electric motor itself, as well as the entirety of electric power steering system, should have its durability elevated and electric power consumption decreased.
The present invention has been made to successfully achieve such desideratum in conventional electric power steering systems of the type described.